Aspects of the invention firstly relate to a vessel with mooring system, comprising a turret anchored to the seabed and a turret casing being part of the vessel, wherein the turret and turret casing are interconnected by a connecting element comprising a bearing arrangement and wherein the connecting element has a first end connected to the turret casing and an opposite second end connected to the turret.
It is noted that although in the present description the indication “vessel” is used, this expression is not intended to restrict the scope of the present invention to ships or boats, but extends to a wide variety of devices floating on the surface of the sea, such as but not limited to buoys or floating production facilities.
A mooring system is used for mooring the vessel while allowing a rotation thereof, such that the vessel can weathervane for assuming a position in which the loads on the mooring system (but also on the vessel) are minimised.
The turret defines a substantially geostatic part which, for example, may be anchored to the seabed using anchoring lines. The turret casing, which often (in case of an internal mooring system) is integrated in a so-called moonpool at a forward part of the vessel (but which also could be part of an outrigger extending beyond the hull of the vessel, and thus defines an external mooring system) defines a part of the vessel which will move therewith, and thus relative to the (geostatic) turret. The connection between the turret and the turret casing therefore is defined by a connecting element which comprises a bearing assembly providing for said rotating connection between the turret casing and the turret.
For a proper operation of such a mooring system it is required that the bearing assembly maintains its function (i.e. allowing a relative rotation between the turret and turret casing) under all circumstances. Thus it is important to prevent deformations of the hull of the vessel (as may or surely will occur under influence of, for example, the waves) from being transferred to the bearing arrangement (or, oppositely, to prevent deformations of the turret from being transferred to the bearing arrangement), thus preventing a detrimental deformation of the bearing assembly (which might lead to a locking thereof).
It is noted that, although here the prevention of deformations is mentioned, it should be kept in mind that essentially it only is required to limit deformations to a level at which the proper operation of the bearing arrangement is not negatively influenced.
For preventing an undesired deformation of the bearing assembly basically two types of designs are known to date. Firstly, the design of the “torsion-box” type utilises a very stiff structure (torsion-box) surrounding the bearing (for example integrated into the hull or turret casing of the vessel when the bearing arrangement is positioned at the first end of the connecting element). Thus, deformations of the hull of the vessel cannot or hardly be transferred to the bearing assembly (neither deformations from the turret) because the torsion-box cannot or hardly be deformed. Secondly, the design of the “cone” type utilises as a connecting element a rather flexible truncated cone-like construction which with its wider base is connected to the vessel (i.e the turret casing) and the narrower top of which supports the bearing assembly which is connected to the turret. Due to the inherent flexibility of such a truncated cone this design substantially prevents deformations of the hull of the vessel from being transferred to the bearing assembly (instead, the cone itself will deform while the bearing assembly maintains its original circular shape). However, when using such a cone there is dilemma in its design. On one hand the cone should be sufficiently flexible for allowing its deformation upon a deformation of the hull of the vessel. On the other hand, however, the cone should be sufficiently strong (stiff) to prevent it from collapsing (buckling) due to the considerable loads (primarily the vertical loads such as the mooring and riser loads and the weight of the turret with all its components) exerted at its top (through the bearing assembly).